It is a common manufacturing practice to secure nuts directly to sheet metal pieces through a welding process. In the manufacture of motor vehicles today, there are virtually hundreds of applications for such "weld nuts" including various forms of bracketry, air bag supports, door parts and many others.
A variety of welding assemblies have been utilized for the production of weld nuts. One of the most common assemblies employs upper and lower welding units and a retractable steel locating pin. See, for example, U.S. Pat. No. 2,731,535 issued to Grey.
As discussed by Grey, in operation, a sheet metal piece with a preformed aperture is loaded on top of the lower welding unit such that the aperture is aligned directly over the locating pin recessed within the lower welding unit. The locating pin is then extended through the aperture of the sheet metal piece to capture a nut fed from a weld nut feeder positioned above and to one side of the locating pin. The nut is then centered about the locating pin over the aperture by the force of gravity. The upper welding unit then descends over the nut and a voltage differential is applied between the upper and lower welding units to fuse the nut to the sheet metal piece. Finally, the upper welding unit is withdrawn, the locating pin is retracted into the lower welding unit, and the sheet metal piece is removed from the welding assembly.
Significantly, this conventional type of welding assembly is known by those skilled in the art to suffer from two basic problems:
First, the electrical conductivity of the steel locating pin frequently causes arcing between the sheet metal workpiece and the locating pin during the actual welding process. As a result, the locating pin often becomes pitted and coated with a slag material that interferes with the extension and retraction of the locating pin within the lower welding unit. Eventually, the locating pin becomes inoperable and must be replaced, resulting in down time for the welding assembly, lost production, and increased labor. In fact, conventional steel locating pins generally require replacement on the order of once every eight hours.
Second, the design of the cavity within the lower welding unit in which the locating pin is housed often does not allow the tip of the locating pin to recede below the surface of the lower welding unit. As a result, the tip of the locating pin can occasionally collide with sheet metal pieces as they are loaded onto the lower welding unit. Repeated or particularly forceful collisions with sheet metal pieces result in the deformation of the tip of the locating pin. Such deformation may reduce or eliminate the effectiveness of the locating pin in capturing and retaining nuts fed from the weld nut feeder, once again necessitating replacement of the locating pin resulting in down time, lost production and additional labor. Constructing the locating pins from steel lessens this problem to some extent, but adds to the arcing problem discussed previously.
U.S. Pat. Nos. 2,623,974 and 2,903,562 issued to Prucha and Emmons et al., respectively, illustrate two attempted solutions to the arcing problem described above. U.S. Pat. No. 2,623,974 issued to Prucha discloses a partially insulated locating pin. Similarly, U.S. Pat. No. 2,903,562 issued to Emmons et al. discloses a locating pin in combination with a partially insulated metallic plunger. As disclosed by Emmons, the plunger acts to remove the locating pin from contact with the nut and sheet metal piece during the actual welding process. While both theses approaches serve to reduce arcing associated with the locating pin during the welding process, neither completely eliminates the problem. Indeed, arcing may still and often does occur over the air gaps between the uninsulated portions of the locating pin or plunger and the surfaces of the nuts and sheet metal pieces. Moreover, neither of these devices address the problem of physical deformation of the locating pin caused by collisions with the sheet metal pieces as they are loaded onto the lower welding unit.
U.S. Pat. Nos. 3,219,790 and 4,609,805 issued to Johnson and Tobita et al., respectively, disclose specialized electrodes as part of the upper and lower welding units that eliminate the need for a locating pin entirely. U.S. Pat. No. 3,219,790 issued to Johnson discloses upper and lower welding units adapted to orient and guide a nut directly over a sheet metal piece before welding the two together. However, this device requires specialized nuts to function properly. Additionally, it generally increases the costs associated with a welding assembly by virtue of its specialized electrodes and more complex method for feeding nuts into position for welding.
U.S. Pat. No. 4,609,805 issued to Tobita et al. also discloses specialized electrodes adapted to guide and hold a nut onto a sheet metal piece, as well as punch a hole in the sheet metal piece over which the nut is to be welded. This device also discloses insulating material to partially isolate a punch housed within the lower welding unit from the actual electrode of the lower welding unit. Once again, however, the Tobita device requires specialized nuts having grooves therein that cooperate with the punch housed in the lower welding unit. Additionally, the device will generally increases the costs associated with a welding assembly by virtue of its specialized electrodes and more complex method for feeding nuts into position for welding. Moreover, the problem of arcing may still occur over the air gap between the uninsulated portions of the punch and the lower electrode. Still further, Tobita discloses that the hole is punched from the sheet metal piece only after the nut and the sheet metal piece have been welded together. Thus, this device suffers from an inherent inefficiency since part of the weld joint must be broken to properly form the hole on the sheet metal piece.